This invention relates to the tanning of animal hides and in particular to the more efficient utilization of chromium tanning compositions.
This application is related to the following copending applications: Ser. No. 052,149, filed June 26, 1979, now U.S. Pat. No. 4,285,689, in the name of Marcel Siegler, entitled "Leather Tanning Composition and Method"; and Ser. No. 052,217, filed June 26, 1979, now U.S. Pat. No. 4,270,912, in the name of William C. Prentiss, entitled "Stabilized Tanning Composition and Method."
Chromium compositions are universally employed as primary tanning agents in the manufacture of leather from animal hides. A typical chromium composition (popularly known as and hereinafter abbreviated to "chrome tan" or "chrome") is a chromium salt solution prepared by reducing sodium bichromate with a sugar or a sugarlike reagent and sulfuric acid. The resulting chrome tan is a basic chromium sulfate, Cr(OH)SO.sub.4, having chromium in the trivalent state and having about 25-60% basicity, more usually about 33-40% basicity. Chrome tans have numerous advantages over other tanning agents such as vegetable extracts, synthetic tanning agents (such as phenolic resins) similar in tanning action to vegetable extracts, aldehydes such as formaldehyde and glutaraldehyde, and other mineral tanning agents such as aluminum, iron, titanium and zirconium salts. These advantages include production of a leather more resistant to collagen denaturation and greater control over the tanning process. Chromium salts are also sometimes used in secondary tanning treatments, such as pretanning and retanning (see U.S. Pat. No. 3,888,625), although it is more common to use other tanning agents for such purposes.
The complete tanning process generally comprises both wet and dry operations. The major steps of the wet operations are unhairing (including liming), bating (removal of unhairing chemicals and non-leather making substances), pickling (acidification to maintain the subsequently added chrome tan soluble and addition of brine to prevent acid swelling), and chrome tanning. Secondary wet operations often following the chrome tanning include wringing, splitting and shaving, retanning, coloring, fatliquoring, and setting out. Dry operations follow the wet operations and include drying, conditioning, staking, buffing, finishing, plating, measuring and grading. These and other leather processing technique are thoroughly described in the literature, as in "Leather Facts," New England Tanners Club, Peabody, Mass. (1965).
In chrome tanning, it is known that the chromium may be complexed or "masked" with an organic acid such as formic or acetic acid to increase the pH at which hydrated chromium oxide begins to precipitate from the basic chromium sulfate to above the pH at which the carboxyl groups of the hide collagen begin to ionize and become more receptive to attachment of the chrome complex. Thus, it is known that chrome tanning should begin at a fairly low pH (for example, below about 3.0) to permit the rapid penetration of chrome through the pickled hide, and the tanning should be finished at a higher pH such that the chrome can combine with the collagen and form hydrated chromium oxide-sulfate cross-linked microstructures which provide the stabilized condition in the hide known as "tanning." Formate-masked chrome generally requires a pH of about 3.75 to release the chrome while acetate-masked chrome requires a pH of about 4.25. The pH adjustment, however, should be short of that which will cause precipitation of hydrated chrome oxide since the latter does not play a part in the tanning process and can lead to poor tanning and undesirable side effects.
While there are many variations on chrome tanning, for the purpose of shortening process time, increasing rate of chrome penetration and improving chrome utilization, the variations are for the most part based on pH and temperature control, masking of the chrome, and concentration of chrome in the tanning liquor ("float"). The latter is usually controlled by adding or subtracting water during the tanning process.
Usually, the amount of chrome tan charged is about 1.5% to about 2.0% calculated as Cr.sub.2 O.sub.3, based on "white weight" (weight of water-swollen hide stock after unhairing and liming but before bating). At the end of tanning, the chrome left over in the tanning liquor usually varies from about 6.0 to about 12.0 g./l. chrome tan as Cr.sub.2 O.sub.3 as determined by American Leather Chemists Association ("ALCA") Test Method C-1. This exhaust liquor contains chrome in the form of a highly masked, soluble, hydrated, chromium oxide-sulfate complex, and is probably more anionic then cationic in character. At equilibrium about 2.5 to 4.0% chrome tan as Cr.sub.2 O.sub.3 commonly is fixed in the hide stock on a moisture free basis, as determined by ALCA test method D-10. However, this amounts to only about two-thirds of the chrome being utilized, the balance being discarded in the exhaust liquor.
Although chrome in the trivalent state is nowhere as toxic as hexavalent chrome, it is considered hazardous by regulatory agencies when found in effluent streams. Reducing the amount of chrome in the effluent by increasing the amount of chrome fixed in the hides, that is, improving the efficiency of chrome utilization in chrome tanning, has been the subject of many proposals in recent years. Among the anti-pollution measures practiced may be mentioned re-use of exhaust chrome liquors in pickling and/or tanning, and precipitation of the chrome followed by redissolving and reuse. It is estimated that only about 60% of the chrome in the effluent is recovered for re-use by these procedures, since it is difficult to recover all of the chrome liquor remaining from the tanning step and subsequent processing. Accordingly, recent antipollution measures have concentrated on reducing chrome in the exhaust liquor by improving chrome utilization during the tanning.
A number of chemical methods have been developed to improve the efficiency of chrome utilization. One such method is the use of sparingly soluble neutralizing agents for the acidified chrome tanning bath, such as magnesium oxide and calcium carbonate, rather than readily soluble sodium bicarbonate. By gradually increasing the pH of the float through slow solubilization, and thereby gradually increasing the basicity of the chrome, more chrome is utilized in the tanning process (by avoiding the normal levels of precipitation) than would be the case with rapidly soluble salts. Considerable care is required with these neutralizing agents, however, because with temperature changes, solubility of the neutralizing agent changes and resultant precipitation of chrome within the stock may occur, leading to spots in the leather. Nevertheless, at the higher pH achievable by this technique, the chrome in the exhaust liquor can be reduced to less than one g./l. Cr.sub.2 O.sub.3 by reducing the chrome charge to 80% of normal. A variation on this process is disclosed in U.S. Pat. No. 3,888,625 wherein exhaust liquor is neutralized by reaction with a sulfite salt and an aldehyde or aldehyde-generating compounds (such as certain oxazolidines).
It is also known to use sparingly soluble dicarboxylic acids, such as adipic or phthalic, as masking agents to increase chrome exhaustion. More soluble acids, such as oxalic, malonic and maleic, are also known to reduce chrome exhaustion. However, it is difficult to achieve uniform penetration of the chrome tan through full thickness hides when using these techniques. The coordination of chrome with alkaline materials such as triethanolamine is also useful for the dechroming of scrap leather. See R. M. Lollar, JALCA, 35, (10) 584(1940).
Another known effort to improve efficient use of chrome is the use of aminocarboxylic chelating agents such as ethylenediaminetetraacetic acid or salts thereof, as in West German Pat. No. 1,257,352 granted July 18, 1968. In this approach a preformed chrome tan chelate is prepared with the chelating agent at a low pH where it is stable. The chelate is then applied to limed hide stock or hide stock washed out of bate and still alkaline, at which pH the chrome chelate becomes unstable and tanning takes place with the formation of the calcium chelate reaction product (from lime still in the bated stock). Because of the resultant high pH, there will be less chrome in the exhaust liquor. However, again because of the high pH, it is difficult to achieve good penetration and uniform distribution of the chrome tan in the hide.
Because of the multiplicity of steps and complexity of the total leather manufacturing process, the tanning industry is reluctant to make any significant change in a key step, such as the chrome tanning step, in view of the possibility that such change will upset their long established control over the other steps and therefore the quality of the leather. Accordingly, it has been the tendency in the industry to recycle or precipitate effluent chrome rather than to make changes in the chrome tanning step. Precipitation and recycling of course, substantially add to the complexity and cost of the total process. The industry most likely, therefore, would find acceptable an improvement which would reduce chrome in the effluent so long as no significant changes are required in the chrome treatment step and the quality of the leather is not lessened.